Everything You Need to Know about Narcissists, Psychopaths, and Abuse - click on this link: http://www.narcissistic-abuse.com/faq1.html
Parents of codependents teach their offspring to expect only conditional, transactional love: the child is supposed to render a service or fulfil the parent's wishes in return for affection and compassion, attention and emotion. Ineluctably, the hurt child reacts with rage to this unjust mistreatment.
With no recourse to the offending parent, this fury is either directed outwardly, at others (who stand in for the bad parent) - or inwardly. The former solution yields a psychopath, or a passive-aggressive (negativistic) - the latter solution, a masochist. Similarly, with an unavailable parent, the child's reserve of love can be directed inward, at himself (to yield a narcissist), or outward, towards others (and, thus, form a codependent.)
All these choices retard personal growth and are self-annihilating. In all four paths the adult plays the dual roles of a punitive parent and an eternal child, who is unable and unwilling to grow up for fear of incurring the wrath of the parent with whom he had merged so thoroughly early on.
When the codependent merges with a love object, she interprets her newfound attachment and bond as a betrayal of the punitive parent. She fully anticipates the internalized parent’s disapproval and dreads its (self-)destructive disciplinarian measures. In an attempt to placate this implacable divinity she turns on her partner and lashes out at him, thus establishing where her true loyalties and affiliation lie (i.e., with the parent.) Concurrently, she punishes herself as she tries to pre-empt the merciless onslaught of her sadistic parental introjects and superego: she engages in a panoply of self-destructive and self-defeating behaviours.
Acutely aware of the risk of losing her partner owing to her abusive misconduct, the codependent experiences extreme abandonment anxiety. She swings wildly between self-effacing and clinging (“doormat”) behaviours on the one hand and explosive, vituperative invective on the other hand: the former being the manifestations of her “eternal child” and the latter expressions of her “punitive parent”.
Such abrupt shifts in affect and conduct are often misdiagnosed as the hallmarks of a mood disorder, especially Bipolar Disorder. But where Dependent Personality Disorder is diagnosed, these pendular tectonic upheavals are indicative of an underlying personality structure rather than of any biochemically-induced perturbations.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 3 DVDs with 16 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)